The use of magnesium alloys, aiming at reducing the weight of vehicles, is growing from year to year due to a number of their particularly advantageous properties, such as low density, high strength-to-weight ratio, good castability, easy machinability and good damping characteristics. Most of this growth has been associated with interior parts made of commercial magnesium alloys of AZ and AM families, that can operate only at temperatures up to 100° C. and therefore cannot be used for powertrain components that should serve at temperatures up to 150-175° C. The main problems in expanding the use of Mg alloys in the transportation industry are associated with their creep behavior, castability, corrosion behavior, and the costs.
Commercial die casting magnesium alloys of Mg—Al—Zn system, such as AZ91D, and of Mg—Al—Mn system, such as AM50A and AM60B exhibit good castability, improved corrosion resistance, and attractive mechanical properties at ambient temperature. However, the above alloys exhibit insufficient elevated temperature strength, poor creep resistance, and poor bolt load retention properties. Therefore these alloys can serve only at temperatures lower than 110° C. Recently several creep resistant magnesium alloy have been developed based on Mg—Al—Ca, Mg—Al—Sr, Mg—Al—Ca—Sr, Mg—Al—Sr—RE, and Mg—Al—Ca—Re alloying systems. It should be noted that the use of alkaline earth elements like Ca and Sr requires the Al content not less than 6% in order to avoid sticking to die and increased susceptibility to hot cracking. However, increased Al content results in the deterioration of creep resistance and thermal conductivity—two very important properties for the implementation of Mg alloys as housings for LED lighting devices, the application that has been penetrating the automotive industry at unprecedented rate for the last five years. For such applications, creep resistant magnesium alloys based on Mg—Al—RE alloying system can be considered as promising candidates. Several creep-resistant die casting Mg—Al—RE alloys have been developed and described. FR 2090881 and DE 2122148 relate to a magnesium alloy comprising 0.9-6.5 wt. % Al, 0.24-10 wt. % RE, 0-1.5 wt. % Mn, and common impurities wherein RE elements are employed as Ce-based mischmetal containing 50-60% Ce, 15-30% La, and the rest Didymium, which is usually a 3 to 1 mixture of Nd and Pr. U.S. Pat. No. 6,467,527 relates to a die casting process for a magnesium alloy comprising 1-10 wt. % Al, 0-1.5 wt. % Mn, and at least one alloying element selected from 0.2-5.0 wt. % RE metal, 0.02-5.0 wt. % Ca, and 0.2-10.0 wt. % Si. WO2005/108634 describes magnesium alloy comprising 1-10 wt. % Al, 1-8 wt. % RE elements wherein 40% or more of RE elements is Ce, 0-0.5 wt. % Mn, 0.0-1.0 wt. % Zn, 0-3.0 wt % Ca, and 0.0-3.0 wt. % Sr. EP 1957221 discloses die casting process of a magnesium alloy comprising 2.0-6.0 wt. % Al, 3.0-8.0 wt. % RE elements wherein 40% or more of RE elements is Ce, 0.0-0.5 wt. % Mn, 0.0-1.0 wt. % Zn, less than 0.01 wt. % Ca, less than 0.01 wt. % Sr, and the balance are unavoidable impurities. U.S. 2009/0116993 describes magnesium alloy containing 3.0-5.0 wt. % Al, 0.4-2.6 wt. % Ce, 0.4-2.6 wt. % La, 0.2-0.6 wt. % Mn, wherein the total amount of Fe, Cu and Ni impurities is less than 0.03 wt. %. CN 102162053 discloses the preparation of magnesium alloy comprising 3.0-5.0 wt. % Al, 3.5-4.5 wt. % of Ce based mischmetal, and 0.08-0.15 wt. % Ca. CN 102776427 relates to a magnesium alloy containing 3.5-4.4 wt. % Al, 0.17-0.25 wt. % Mn, and 5.5-6.4 wt. % RE elements wherein Ce, La and Nd account for 35-40 wt. %, 60-55 wt. %, and 5 wt. %, respectively. Furthermore, CN 101440450 describes a magnesium alloy comprising 3.5-4.5 wt. % Al, 1.0-6.0 wt. % La, 0.2-0.6 wt. % Mn, wherein the total amount of Fe, Cu and Ni impurities is less than 0.03 wt. %. CN 104046871 discloses a magnesium alloy comprising 3.5-4.5 wt. % Al, 2.5-3.5 wt. % La, 1.5-3.0 wt. Sm, 0.2-0.4 wt. % Mn, wherein the total amount of Fe, Cu and Ni impurities is less than 0.03 wt. %; it should be noted that the presence of the expensive element Sm makes the above invention unpractical and unsuitable for the industrial production.
It is an object of this invention to provide creep resistant magnesium-based alloys being suitable for elevated temperature applications, and showing superior energy absorption properties and good performance in the corrosive environment.
It is another object of the present invention to provide a process for preparing ingots of the above alloys.
It is a further object of the present invention to provide alloys that are especially well suitable for high-pressure die casting process, and which enable high casting rate.
It is a still further object of this invention to provide alloys which have low susceptibility to hot cracking and sticking to die.
It is also an object of this invention to provide alloys which have enhanced thermal conductivity.
It is also another object of this invention to provide alloys with improved bearing and shear properties at ambient and elevated temperatures.
It is further an object of this invention to provide alloys which demonstrate the aforesaid behavior and properties at an affordable cost.
Other objects and advantages of present invention will appear as description proceeds.